A flash bulb type thermal copying device uses a flash light source typically consisting of a xenon discharge bulb, and the flash light produced by the flash light source is radiated upon an assembly of an original and a heat sensitive recording medium which are placed closely one over the other so that the temperature variations corresponding to the original image may reproduce an identical image in the heat sensitive recording medium.
The heat sensitive recording media normally used in such a flash bulb type thermal copying device typically consist of heat sensitive stencil printing master plate sheets, or heat sensitive projection transparencies for OHP, and the flash bulb type thermal copying device is often used as a plate making device for a stencil printing device, or a device for preparing projection transparencies for OHP.
As a flash bulb type thermal copying device which processes a heat sensitive sheet having a large area by using a small number of flash light sources or a flash light source of a limited intensity, there is known the flash bulb type thermal copying device of the divided exposure type, for instance the one disclosed in Japanese patent laid open publication No. 60-161150 in which a laminated assembly consisting of an original sheet and a heat sensitive recording medium laid one over the other, and a light radiating surface unit including a flash light source are incrementally moved relative to each other by a certain pitch. The flash light source is activated every time the two parts are moved relatively to each other by the prescribed pitch.
In a thermal copying process, it is necessary for the original and the heat sensitive recording medium to be in a thermally close contact with each other during the entire exposure process. In the case of the divided exposure process described above, the original and the heat sensitive medium must stay stationary with respect to each other until the original is entirely copied.
Therefore, according to the conventional flash bulb type thermal copying device, the original and the heat sensitive recording medium are placed in a stationary position, one over the other in close contact by using negative pressure or the like, and the light source is moved relative to this laminated assembly. Alternatively, a base sheet is provided on one side of the heat sensitive recording medium so that the original may be interposed between the base sheet and the heat sensitive recording medium, and the heat sensitive recording medium may be moved relative to the light source along with the base sheet. In this case, a pressure mechanism is required to press the heat sensitive recording medium and the base sheet against each other at the position for exposure by the light source.
However, according to the method of placing the original and the heat sensitive recording medium one over the other in an intimate contact at a stationary position by using negative pressure, and moving a light source relative to this laminated assembly, it is necessary to use relatively large units for placing the original and the heat sensitive recording medium one over the other in an intimate contact, and for moving the light source. Thus, a system based on this method is highly unsuitable for simple flash bulb type thermal copying devices intended for use in homes.
According to the method of moving the laminated assembly consisting of the original and the heat sensitive recording medium, a sufficient reliability may not be attained with regard to the securement of the original and the heat sensitive recording medium relative to each other during each process of exposure. Furthermore, a pressure mechanism is needed for pressing the original and the heat sensitive recording medium against each other for each process of exposure, and the system cannot be made so simple as desired.